Nonalcoholic fatty liver disease (NAFLD) results from excessive accumulation of fat in the liver. By itself, hepatic fat accumulation is not life-threatening, but a significant proportion of NAFLD patients progress to more severe forms of the disease characterized by inflammation, fibrosis, and cirrhosis, a condition known as nonalcoholic steatohepatitis (NASH). Although some factors, such as diet, obesity, insulin resistance, and ethnicity, have been associated with hepatic fat storage, clinical characteristics predicting NAFLD progression to more severe forms of fatty liver disease have not yet been identified. Further, little is known of the underlying physiology governing disease progression, and this gap in knowledge is a barrier to predicting which NAFLD patients will develop fibrosis and cirrhosis. The overall plan for this project, therefore, is to identify factors that predict progression of NAFLD to more severe forms of the disease. The specific goals of this study are to first evaluate the relationship between individual and composite predictors of NAFLD progression to steatohepatitis, fibrosis, and cirrhosis. Next, we will utilize a genome-wide approach to genotype 1M markers in 2075 obese individuals and assess association between these markers and NASH severity, as defined by histological grade of hepatic biopsy. All trait-associated markers and haplotypes will be validated in two independent study samples. Finally, we will perform RNA sequencing to measure hepatic gene expression and identify gene networks that are correlated with progressive NASH severity. We will also combine genotype and RNA sequencing data in an innovative approach to identify genetic variants associated with mRNA expression levels in liver samples comprising the entire spectrum of NAFLD stages. Completion of these aims will advance our understanding of NASH development and progression to more severe forms of the disease and may lead to better treatment and prevention strategies for at-risk individuals. PUBLIC HEALTH RELEVANCE: Individuals suffering from obesity, insulin resistance and/or type 2 diabetes often store excessive fat in the liver. By itself, hepatic fat accumulation is not life-threatening, but some affected patients will develop more severe forms of fatty liver disease including steatohepatitis, fibrosis, and cirrhosis;however, no clinical measures are yet available to identify which individuals are most likely to progress. This study will identify genetic variants that modulate severity and progression of nonalcoholic fatty liver disease, which will lead to better treatment and prevention strategies in at-risk individuals.